It has been a long term objective in the printing industry to form images directly from an electronically composed digital database by a "computer-to-plate" system. The advantages of such a system, over the traditional methods of making printing plates, would be:
(i) the elimination of costly intermediate film and processing chemicals; PA1 (ii) a saving of time; and PA1 (iii) the ability to automate the system with consequent reduction in labor costs. PA1 (a) providing a roll of a printing plate precursor comprising: PA1 (b) imagewise exposing the photosensitive layer of said printing plate precursor by means of a roll-fed imagesetter or phototypesetter; and PA1 (c) processing the exposed printing plate precursor to provide hydrophilic non-image areas and oleophilic image areas.
The advent of imagesetters and phototypesetters has increasingly afforded the opportunity of forming an image directly on a printing plate precursor without the need for an intermediate film-making stage. Such devices are employed to output fully paginated text and graphic images at a high resolution, employing various techniques for image creation. Computerized signals are employed for image formation, which enable one to achieve significant improvements in cost, efficiency and quality.
The majority of the existing imagesetters and phototypesetters are roll fed units designed for the exposure of film or paper, rather than less flexible aluminum-based lithographic printing plates. Certain plastic-based printing plates, such as the Mitsubishi Silver Master, can be processed through imagesetters, but these plates have disadvantages of poor dimensional stability, making them unsuitable for full color printing. The plastic-based printing plates also offer poor lithographic properties compared to conventional aluminum-based lithographic plates.
In general, it is found that plastic sheet, such as polyethylene terephthalate, has good flexibility, making it suitable for processing through imagesetters, but suffers from inadequate hydrophilicity, dimensional stability and coating adhesion for lithographic printing plate applications. On the other hand, aluminum sheet, especially when grained and anodized, is suitable for almost all requirements for a supporting substrate of a presensitized lithographic printing plate, giving excellent hydrophilicity, dimensional stability and coating adhesion. Aluminum does, however, have the disadvantage of limited flexibility.
Thus, there is an ongoing need for a process for making high quality lithographic printing plates using roll-fed imagesetters or phototypesetters.